MLA

Browne, John, approximately 1608-1691. [Letter of thanks to the Lord General, Sir Thomas Fairfax, sent at the command of the House of Lords, 1641 Aug 17]. Box 1. [n.d.]. https://collections.library.yale.edu/catalog/31906498.

APA, 6th edition

Browne, John, approximately 1608-1691. (["Box 1"])([n.d.])[[Letter of thanks to the Lord General, Sir Thomas Fairfax, sent at the command of the House of Lords, 1641 Aug 17]]. https://collections.library.yale.edu/catalog/31906498.